1. Field of the Invention
This invention relates to polycarbonate compositions endowed with very high impact properties, without loss in tensile and flexural properties. In particular the present invention relates to high impact compositions based on aromatic polycarbonate derived from the polycondensation of a carbonic acid precursor with a phenol having at least two phenolic hydroxyl groups.
2. Discussion of the Background
Engineering plastics are increasingly used in the world market as new technologies and uses increase the need for better and more specialized materials. Aromatic polycarbonates, in particular, have grown to a wide number of different applications in several fields.
Aromatic polycarbonates are known to be tough materials especially when in thin sections. However, their impact resistance decreases drastically when the thickness of an article exceeds a critical value, usually between 5 and 6 mm, depending on the geometry of the article and the molecular weight and chemical composition of the polycarbonate. Moreover, most recent potential applications of polycarbonates show that improved toughness of these materials would be desirable at all thicknesses.
There have been many attempts to produce tougher polycarbonates either by chemical modification of the polymer chain or by addition of other materials both polymeric and non-polymeric. The latter include thermoplastic additives or elastomeric modifiers. In recent years materials known as rubber interpolymer composites or "core-shell" latex particles have been used successfully to impact modify engineering thermoplastics.
Core-shell materials have been disclosed in many publications over the past ten years. Acrylic core-shell polymers and the preparation thereof are described in U.S. Pat. Nos. 4,096,202, 3,864,428 and 4,200,567. The use of acrylic core-shell polymers as additives for improving the impact resistance of polyester/polycarbonate blends is described in U.S. Pat. No. 4,897,448.
U.S. Pat. No. 4,299,928 describes a polycarbonate composition in which an acrylic core-shell impact modifier is used. The rubber modifier of this disclosure consists of a rubber core fully covered by a shell. The core is made from an alkyl acrylate such as butyl acrylate, copolymerized with a multifunctional monomer, with reactive groups that polymerize substantially at the same polymerization rate, such as butylene diacrylate. The shell consists of a glassy polymer such as methylmethacrylate, styrene or acrylonitrile. Furthermore, the shell is grafted onto the core with the aid of a grafting monomer such as allyl acrylate. Although an improvement in the impact properties of the polycarbonate is reported, this appears to be still insufficient for many applications especially at reduced use temperatures. The room temperature thick section notched Izod impact properties were shown to improve with the addition of the core-shell impact modifiers, however, the improvement of low temperature thick section notched Izod impact properties was not demonstrated. Improvement of these low temperature properties is essential in many commercial applications of this class of materials.
Multiphase composite materials have been studied throughout the last ten years, with particular attention to their morphology. As the core-shell materials are a biphasic system, their morphology depends largely on the ratio of the various components and the way they are reacted together. Most commonly used core-shell materials are a lightly crosslinked rubbery core covered by a continuous layer or shell of a more rigid material which is usually grafted or entangled on the surface of the core. The thickness of the shell depends on the relative weight ratios of the two phases. Other morphologies have also been reported such as semicontinuous, spotted and inverted core-shell polymers, but such materials have not been used as impact modifiers. See J. W. Vanderhoff, et al., Scientific Methods for the Study of Polymer Colloids and Their Applications, Candau and Ottewill (eds.), 529 (1990); Polymer Latexes: Preparation, Characterization, and Applications, ACS Symposium Series No. 492, Daniels, Sudhol, El-Aasser, (eds.), (1992); Y-C. Chen, et al, J. Appl. Polym. Sci. 42, 1049 (1991); Y-C Chen, et al., Polymer International 30, 185 (1993). In particular, there is no mention in the literature of the use of such materials in admixture with polycarbonate resins.